1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image by developing an electrostatic latent image formed on a photosensitive member, and is applied to a copier, a printer, or the like.
2. Description of the Related Art
Conventionally, methods for electrostatically charging a photosensitive member on which an electrostatic latent image is formed before an image which is to be transferred to a recording sheet in an image forming apparatus is developed include a corona electrostatic charging method. The corona electrostatic charging method is a method that a high voltage of about 5 kV to 10 kV is applied to a metallic wire with a diameter of about 50 μm to 100 μm placed inside a corona electrostatic charger disposed in opposing relation to a photosensitive member, and an air layer between the photosensitive member and the corona electrostatic charger is ionized by corona discharge to electrostatically charge the photosensitive member.
With repeated discharge, stains become attached to the metallic wire itself to deteriorate discharging performance, and hence it is necessary to clean and replace the metallic wire on a regular basis. Moreover, corona products are produced due to ozone generated by corona discharge, and a surface of the photosensitive member tends to gradually absorb moisture due to the corona products.
Also, ozone products are generated due to reaction of ozone generated due to corona discharge and moisture in the air, and become attached to the photosensitive member tending to absorb moisture. The resistance of the surface of the photosensitive member decreases due to the ozone products, and image deletion (such as image blur or distortion) occurs due to insufficient electrostatic charging of the photosensitive member when an electrostatic latent image is formed on the photosensitive member.
The techniques described hereafter have been proposed so as to prevent such image deletion. They include the technique that moisture on the surface of the photosensitive member is removed by constantly heating the photosensitive member using a heater, and the technique that a shielding member is disposed between the photosensitive member and an electrostatic charger so as to prevent ozone products generated in the neighborhood of the metallic wire from becoming attached to the photosensitive member and its vicinity. It should be noted that cloth or the like is used as the shielding member.
The technique described hereafter has also been proposed so as to prevent such image deletion (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2007-072212). According to Japanese Laid-Open Patent Publication (Kokai) No. 2007-072212, during an energy-saving mode in which power consumed by an image forming apparatus is saved, a shielding member is disposed at a shielding position between a photosensitive member and an electrostatic charger instead of heating the photosensitive member by a heater. This prevents image deletion while cutting down on power consumed by the image forming apparatus as compared to a case where electric current is constantly passed through the heater.
However, according to the above described techniques, for example, when the user turns on a switch that activates the heater for the photosensitive member during the energy saving mode in which the shielding member is disposed at the shielding position between the photosensitive member and the electrostatic charger, the photosensitive member is heated with the shielding member being disposed at the shielding position. In this case, the temperature of the shielding member increases because it is fanned by heat generated from the heater for the photosensitive member, and this brings about deterioration of the shielding member.
As a result, the shielding member cannot normally work, causing a phenomenon such as image failure. For example, when the shielding member is hardened by heat, and cannot normally move from a shielding position to the retracted position, the shielding member is left inside an image-forming region to inhibit the photosensitive member form normally working and thus cause a phenomenon in which, for example, an image is partially chipped.